1. Field of the Invention
This invention relates to a method of and an apparatus for detecting the alignment accuracy in an exposure apparatus for transferring the pattern image of a mask, a reticle or the like onto a photosensitive substrate.
2. Related Background Art
In recent years, numerous reduction projection type-exposure apparatuses have been used in the manufacturing line of super-LSI. The role of an exposure apparatus is to make a resist pattern of predetermined line width at a predetermined location on a photosensitive substrate such as a silicon wafer or a glass substrate. To achieve such role, an alignment technique of high accuracy and a reduction projection lens of high resolution are required. In the case of the exposure apparatus, the element concerned with the alignment can be broadly divided into the alignment of the image of a reticle on which a circuit pattern is formed relative to the coordinate system in the exposure field of a projection optical system and the alignment of the exposed image relative to the coordinate system concerned with the arrangement of a plurality of chips formed on a wafer by the feeding mechanism of a wafer stage on which the wafer is placed and an alignment system. The error of the alignment of the image relative to the coordinate system in the exposure field is attributable to the magnification error of the projection optical system, the distortion of the optical system and the rotation error of the reticle mounted on a reticle holder. Also, the error of the alignment relative to the coordinate system concerned with the arrangement of the chips is attributable to the stepping error of the stage, the alignment error, the rotation error of the wafer stage, etc., and further, where global alignment of the wafer is effected, it also includes the scaling error of the stage the error of orthogonality, the error of straightness, etc. Any of these alignment errors may be a cause which reduces the accuracy of the superposition exposure for forming a plurality of layers on the wafer while superposing them upon one another. So, heretofore, in order to examine the superposition accuracy of the exposure apparatus, two reticles having actual circuit patterns or reference marks have been used to effect superposition exposure on a wafer for trial printing and that wafer has been developed, whereafter the resist pattern remaining on the wafer has been measured by the use of a separate examining device of high accuracy, whereby the superposition accuracy has been found. In this case, the use of the examining device of high accuracy is requisite. Such an examining device is expensive and, unlike a manufacturing apparatus such as a stepper, it is not operated at all times. However, where a semiconductive element manufacturing line using a stepper is to be made, an examining device belonging to the line is necessary, and this has meant a high cost. Further, where the accuracy during superposition exposure is to be controlled on the basis of the superposition error measured by the examining device, the work has been very cumbersome and not only the accuracy of the exposure apparatus but also the accuracy of the examining device had to be controlled.
Furthermore, two reticles have been required during the superposition exposure by trial printing, and the arrangement error between the patterns formed on the two reticles, the error of the line width thereof and the reticle alignment error accompanying the replacement of the reticles have been included in the superposition accuracy, and this has also led to a disadvantage that the original superposition accuracy of the exposure apparatus cannot be accurately examined.